The present invention relates to a zinc oxide-based transparent conductor having zinc oxide as its primary component, and a sputtering target for forming the foregoing transparent conductor. Incidentally, the term “transparent conductor” as used herein includes a transparent conductive film.
Today, the material that is being used most as a transparent electrode of flat panel displays and the like is ITO (Indium Tin Oxide), which is obtained by doping indium oxide with an appropriate amount of tin.
The reason why ITO plays the leading part in a transparent conductor is that the various characteristics of ITO such as low resistivity and high transmittance in the visible light region, which are required in a transparent conductor, are superior in comparison to transparent conductors prepared from other materials.
Nevertheless, In (indium) as the raw material to be used in ITO entails problems in that the cost of the end product will increase because indium is expensive, and the supply of materials may become impossible due to resource depletion because indium is a scarce resource. Although the development of a zinc oxide-based transparent conductor having zinc oxide as its primary component is being actively pursued as the development of the substitute material of ITO, there is still a problem in that the resistivity is significantly high in comparison to ITO.
The reason for this is that the conventional development policy of zinc oxide-based transparent conductive materials was only based on the search of an optimal single dopant. In other words, the foregoing policy is to find an element from a periodical table that will serve as an n-type dopant and emit electrons as a result of doping the zinc oxide as the parent material with such element. Specifically, for example, in most cases, a target doped with a candidate element having an atomic valence that is greater than bivalence, which is the valence of zinc, in an appropriate range of concentration is prepared, and this is subject to sputter-deposition to evaluate the resistivity of the film.
As a result of this development policy, although candidate dopants having an atomic valence of trivalence (refer to Patent Document 1) and tetravalence (refer to Patent Document 2) were discovered, the actual condition is that the resistivity of such candidate dopants is far inferior to ITO.
Meanwhile, it has been reported recently that a zinc oxide-based transparent conductor with low resistivity was developed by applying the so-called co-doping theory (refer to Patent Document 3). The subject matter thereof merely requires that an n-type dopant having a concentration above a specified level is contained in an amount that is greater than a p-type dopant.
As it stands now, there have been no other reports in the past in addition to the foregoing report which actually succeeded in preparing a zinc oxide-based transparent conductor with low resistivity, though they have simply satisfied the foregoing requirement.
In addition, although the foregoing report refers to the Metal Organic Chemical Vapor Deposition (MOCVD) method and Molecular Beam Epitaxy (MBE) method in the examples as the method of manufacturing a zinc oxide-based transparent conductor, both of these methods are inappropriate for preparing a large-area transparent conductive film.
[Patent Document 1] Japanese Patent Laid-Open Publication No. S61-205619
[Patent Document 2] Japanese Patent Laid-Open Publication No. S62-154411
[Patent Document 3] Japanese Patent Laid-Open Publication No. 2002-50229